Driver circuits for display devices with spurious glow eliminating circuit



Apnl 28, 1970 J. A. OGLE 3,509,420

DRIVER CIRCUITS FOR DISPLAY DEVICES WITH SPURIOUS GLOW ELIMNATING CIRCUIT Filed May 2, 196s O 2v OO 5 QOAj INVENTR.

James A, Ogle ATTORNEY United States Patent O U.S. Cl. 315-135 9 Claims ABSTRACT OF THE DISCLOSURE The disclosure is of a circuit for operating one or more cathode glow indicator tubes and includes a separate driver, a transistor, coupled between a power supply and the anode of each indicator tube to be operated. Various diodes are coupled to the anode driver transistors for providing the necessary bias voltages, and a single common variable resistor provided in the emitter circuits of all of the transistors can be used to control and vary the current through each transistor as it operates its indicator tube. Means are also provided for holding off the transistors and their tubes as required during quiescent periods to prevent spurious cathode glow.

BACKGROUND THE lINVENTION The present invention relates to circuits for driving cathode glow indicator tubes of the type known as Nixie tubes, which are made by Burroughs Corporation. These tubes comprise a 4gas-filled envelope which contains an anode electrode and a plurality of cathode electrodes which are in the form of letters, numerals, or the like, and which exhibit cathode Iglow when proper potentials are applied between them and the anode.

In recent years, Nixie tubes have been used in instruments, such as calculators, which usually utilize a recirculating or sequentially addressed memory. In such instruments, provisions must be made for holding off the indicator tubes during periods when they are not energized and displaying a number. Usually, if such provision is not made, spurious cathode glow may result; that is, some or all of the cathodes in the display tubes Iwill glow slightly. The present invention provides a display circuit in which such spurious glow is prevented and overall operation of the indicator tubes and the display of information is improved. The circuit has an added ladvantage of being compatible with integrated circuits.

SUMMARY OF THE INVENTION Briefly, circuits embodying the invention include one or more glow cathode indicator tubes and a separate anode driver coupled to the anode of each indicator tube. These anode drivers cooperate 'with similar drivers coupled to the cathodes to operate the tubes. The circuits include means for turning on each anode driver and for holding off the drivers and their tubes during quiescent periods when it is necessary to prevent spurious cathode glow. The circuits include an auxiliary driver, similar to the anode drivers but not connected to a glow tube, for simulating the action of the other drivers and holding off the cathodes, when required, to prevent spurious glow.

DESCRIPTION OF THE DRAWING In the drawing, the single figure is a schematic circuit embodying the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT A circuit embodying the invention may be the display portion of a calculator and includes a plurality of indicator or display tubes (20A, 20B, 20C, etc.) of

3,509,420 Patented Apr. 28, 1970 ice the type described above and represented schematically. Each tube includes an anode electrode 30 and a plurality of cathode electrodes 40 which are shown as numerals and each of which is connected to a driver circuit represented by block 50. A tube 20 usually includes the ten cathode numerals 0 to 9.

The cathodes 40 may be driven in different ways. In one suitable circuit arrangement which is shown, the same cathode numeral in each tube is connected to a common bus S4, to which a driver circuit 50 is coupled. Cathode driving signals .are applied to the tubes serially by circuits 50, and the correct tube and cathode are turned on by the synchronously cycled anode driving circuits to be described. The circuits 50 are well known and are essentially switching circuits which use a transistor for applying operating potential to .a selected cathode, whereby the cathode glows when the corresponding proper potential is applied to the associated anode electrode.

The circuit 10 includes a constant current anode driver circuit for each display tube 20. This circuit includes a positive D.C. voltage supply V1 'which is of the order of 200 volts and is coupled to reference potential such as ground through a Zener diode 70, a lead 80, a diode 90, a lead 100, and a resistor 110. This path from source V1 to ground provides reference or bias voltages for the entire circuit 10.

A separate driver or switching device, a semiconductor device which may conveniently be a three-electrode PNP transistor (120A, 120B, 120C, etc.), is coupled to each anode electrode 30 in tube 20 for selecting and driving a tube 20. Each transistor includes an input or base electrode, an output or collector electrode, and an emitter electrode which is a common or reference electrode. These electrodes are shown in conventional fashion. Each driver transistor 120 has its emitter connected to a bus which is connected in turn through a common variable resistor to power supply V1. The collector of each anode driving transistor 120 is connected by lead 142 directly to the anode 30 of its tube 20 and through resistors to a lead 160 which is coupled to all of the cathode buses 54. These connections prevent excessive voltages from appearing across the transistors 120 during circuit operation. The same result could be achieved in other ways, for example, by connecting each lead 142 through a diode to a positive clamping potential.

The base electrode of each transistor 120 is connected through a resistor to a bus 180 which is connected to lead 80 from the power supply V1. Each base electrode is also coupled through a capacitor and resistor 200 to a terminal 210, which is coupled to a source of negative pulses 220 for turning on the transistor. The turn-on pulses 220 are usually applied to the transistors cyclically and in synchronism with the operation of drivers 50 to turn on each transistor and its tube 20 in turn. The base of each transistor 120 is also connected through a diode 230 to a bus 240 which is connected to lead 100 from the power supply.

According to the invention, in order to avoid spurious glow in the indicator tubes during periods when all of the transistors 120 are turned olf, an auxiliary control means is provided in circuit 10. This means comprises a semiconductor device such as a PNP transistor 250 which has its emitter coupled to bus 130, its collector coupled through a resistor 260 to ground, and its base connected to lead 100. In circuit 10, the auxiliary transistor 250 operates in essentially the same fashion as any of the transistors 120 except that it is not connected to an indicator tube 20.

' In operation of the circuit 10, each transistor 120 drives the anode of a separate display tube 20. In operation of a typical cycling system, such as that found in a calculator, each tube is turned on in sequence and displays a numtion. This causes the emitter bus 130 to assume a po.

tential approximately equal to that on lead 100. The bases of the other transistors 120 which have not received a turn-on pulse assume a potential approximately equal t the potential on bus 180, and, their emitters carry a potential close to that on lead 100, so that these other transistors 120 are reverse-biased and do not turn on. Thus, when one transistor 120 is on and its tube 20` is displaying a number, all of the othertransistors 120 are held off and their tubes 20 cannot glow.

Without transistor 250 in the circuit and with all transistors 120 off and the bases of transistors 120 at a certain bias potential, emitter bus 130 tends to rise in potential until some or all of the transistors 120 conduct and spurious cathode glow is seen in the associated tubes 20. This is prevented in circuit by transistor 250 as follows. If the emitter bus 130 tends to rise in a positive direction, transistor 250 turns on preferentially because its base is at a less positive potential than the bases of transistor 120, and the current in resistor 140I is bypassed through transistor- 250 and resistor 260 to ground. This allows the emitter bus 130` to rise only to the potential of bus 180,

and, with the bases of transistors 120 at about the potential of lead 180, all of the anode drivers 120 remain without forward bias and in the off condition, and no cathode glow occurs.

A similar situation of possible spurious glow exists for a brief time during switching periods when one tube Z0 is switched off and the next is switched on, but the next tube has not yet had time to ionize and glow. During this period, there is no path for current flowing from the source V1 to a tube 20, and this would cause the voltage on bus 130 to rise and one or more transistors 120 to turn on and cause spurious cathode glow. Here again, transistor 250 turns on first, due to its favorable bias arrangement, and draws off the available current and holds of drivers 120.

The purpose of resistor 260 is to limit the current in transistor 250 to the value needed to prevent further voltage rise at bus 130 for any allowed setting of resistor 140. Resistor 110 must provide sufficient base current to transistor 250 so that the above condition is met. Usually, the auxiliary transistor 250 and the anode transistors 120 operate unsaturated since they are operated in current driver mode. That is to say, their base current will be only large enough to maintain the voltage drop across resistor 140. By avoiding saturated operation, the switching speed of transistors 120 is increased.

It is to be noted that the single variable resistor 140 can be used to vary the operating anode driver current in the circuit 10.

What is claimed is:

1. An information display circuit including a power supply coupled through circuit elements to pro vide a source of various bias voltages,

a plurality of display devices,

a separate irst driver circuit coupled between said power source and each of said display devices for operating said devices, each said first driver circuit being adapted to be held ofi or turned on,

a turn-on signal source coupled to each of said iirst driver circuits for applying turn-on signals thereto, and

an auxiliary driver circuit in parallel with all of said first driver circuits and having a common connection therewith,

said first driver circuits and said auxiliary driver cir' cuit being coupled to said source and carrying different bias voltages such that, with no turn-on signal applied to any of said first driver circuits and with said first driver circuits held ofi, said auxiliary driver circuit conducts current preferentially.

2. The circuit defined in claim 1 wherein said display devices comprise gas-filled enevlopes which contain an anode electrode and a plurality of cathode glow character electrodes,

each said first driver circuit being coupled to and operating the anode electrode of its associated display device, and

separate driver circuits coupled to the cathode glow electordes of said device.

3. The circuit defined in claim 1 wherein said display devices comprise gas-filled envelopes which contain an anode electrode and a plurality of cathode glow character electrodes,

each said first driver circuit bein-g coupled to and operating the anode electrode of its associated display device, and

separate driver circuits coupled to the cathode glow electrodes of said devices and synchronized with said first driver circuits to select the proper anode and cathode combinations.

4. The circuit defined in claim 1 wherein said display devices comprise gas-filled envelopes which contain an anode electorde and a plurality of cathode glow character electrodes,

each said first driver circuit including a semiconductor device having an input electrode and an output electrode, the output electrode being coupled to and operating the anode electrode of its associated display devices, and the input electrode being coupled to a source of turn-on pulses, and

separate driver circuits coupled to the cathode glow electrodes of said devices.

5. The circuit defined in claim 1 wherein said display devices comprise gas-filled envelopes which contain an anode electrode and a plurality of catode glow character electrodes,

each said first driver circuit including a semiconductor device having an input electrode, an output electrode, and a common electrode,

the output electrode of each semiconductor device being coupled to and operating the anode electrode of its assocated display device,

the input electrode of each semiconductor device being coupled to a source of turn-on pulses and through different circuit elements, to two buses connected to different ones of said bias voltages,

said common electrodes of all of said semiconductor devices being connected to a common bus which is connected through an impedance to said power supply, and

separate driver circuits coupled to said cathode glow character electrodes for operation in conjunction with said first driver circuits for selection and energization of a particular device and a particular glow cathode.

6. An information display circuit including a power supply coupled to ground through a path including different circuit elements to provide a source of various bias voltages,

a plurality of cathode glow display tubes, each comprising a gas-lilled envelope which contains an anode electrode and a plurality of cathode glow electrodes, each cathode glow electrode exhibiting cathode glow when a suitable first potential is applied to it and a suitable second potential is applied to its anode,

a separate first anode driver circuit coupled between said power source and the anode electrode of each of said display tubes for operating said devices,

each said first driver circuit including a semiconductor device having an input base electrode, an output collector electrode, and a common emitter electrode,

the output collector electrode of each semiconductor device being coupled to and opearting the anode electrode of its associated display tube,

the input base electrode of each semiconductor device being coupled both to a source of turn-on pulses and through a diode to one bus and through a resistive path to a second bus, said first and second buses being connected to different ones of said bias voltages,

said common emitter electrodes of all of said semicomductor devices being connected to a common bus which is connected through an impedance to said power supply,

separate driver circuits coupled to said cathode glow character electrodes for operation in conjunction with said first driver circuits for selection and energization of a particular device and a particular glow cathode,

an auxiliary driver circuit in parallel with all of said first anode driver circuits and having a common connection therewith,

said auxiliary driver circuit being coupled to said source and carrying a different bias voltage than said first anode driver circuits so that with no turnon signal applied to any of said first driver circuits and with said first driver circuits held ofi, said auxiliary driver circuit conducts current preferentially and prevents said first driver circuits from turning on.

7. The circuit defined in claim 6 wherein said auxiliary driver circuit includes a semiconductor device having an input base electrode, an output collector electrode, and a common emitter electrode, said input base electrode being coupled to a point on said path to provide a bias voltage therefor, said output collector electrode being connected to a reference potential, and said common emitter electrode being connected to another point on said path representing another bias voltage.

8. An information display circuit including a power supply coupled through back-to-back diodes and a first resistive path to ground to provide a source of various bias voltages,

a plurality of cathode glow display tubes, each comprising a gas-filled envelope which contains an anode electrode and a plurality of cathode glow electrodes, each cathode glow electrode exhibiting cathode glow when a suitable first potential is applied to it and a suitable second potential is applied to its anode,

a separate first anode driver circuit coupled between said power source and the anode electrode of each of said display tubes for operating said devices,

each said first driver circuit including a semiconductor device having an input base electrode, an output collector electrode, and a common emitter electrode,

the output collector electrode of each semiconductor device being coupled to and operating the anode electrode of its associated display tube,

the input base electrode of each semiconductor device being coupled both to a source of turn-on pulses and through a diode to one bus which is connected to said first resistive path and through a second resistive path to a second bus which is connected to the junction of said back-to-back diodes,

said common emitter electrodes of all of said semiconductor devices being connected to a common bus which is connected through an impedance to said power supply,

separate cathode driver circuits coupled to said cathode glow character electrodes for operation in conjunction with said first driver circuits for selection and energization of a particular device and a particular glow cathode, and

an auxiliary driver circuit in parallel with all of said first anode driver circuits and having a common connection therewith,

said auxiliary driver circuit being coupled to said source and carrying a different bias voltage than said first anode driver circuits so that with no turn-on signal applied to any of said first driver circuits and with said first driver circuits held off, said auxiliary driver circuit conducts current preferentially and prevents said first driver circuits from turning on.

9. The circuit defined in claim 8 wherein said auxiliary driver circuit includes a semiconductor device having an input base electrode, an output collector electrode, and a common emitter electrode, said input base electrode being coupled to a point on said resistive path to provide a bias voltage therefor, said output collector electrode being connected through a resistive path to ground, and said common emitter electrode being connected to said common bus.

References Cited UNITED STATES PATENTS 2,982,880 5/1961 Klipstein 315-169 X 3,183,404 5/1965 Kitz et al 315-169 X 3,227,922 1/ 1966 Glaser et al. 315-169 X 3,340,524 9/ 1967 Rinaldi 315-169 X 3,414,764 1/1968 Kawarnoto et al. 315-135 X 3,119,950 1/1964 Somlyody 315-135 X JAMES W. LAWRENCE, Primary Examiner C. R. CAMPBELL, Assistant Examiner U.S. Cl. X.R. 

